A decoloring apparatus that decolors an image on a sheet, which is printed by decolorable color material, includes a heating section that heats the image at a decoloring temperature or higher and a pressing section that comes into pressing contact with the heating section. As a result, the image of the decolorable color material is heated and pressed at the decoloring temperature thereof or higher by passing the sheet through a nip in which the heating section comes into pressed contact with the pressing section, and the image is decolored. The sheet in which the decoloring is performed is reused as a printing sheet. Decoloring means, in the context of this disclosure, changing the appearance of a colored image comprising a toner or the like, such that the toner or the like which constitutes an image which is visible to a human eye, becomes not readily visible to the human eye.
It is preferable that the image on the sheet be entirely decolored by the decoloring operation, but a portion of the image that is not decolored may remain.
Defects of the decoloring apparatus may be exemplified as one of causes of occurrence of the portions of the image not becoming decolored. As a defect of the decoloring apparatus, for example, a case where the pressure on the sheet carrying the image is not evenly distributed across as the sheet is passed through the nip may occur. If there is a portion in which the pressure of the nip is inadequate, the image coming into contact with that portion of the nip remains colored.
However, since the images, and the location and density of the images, that are printed on the sheets and are subjected to being decolored are generally different on each printed sheet, if sheets passing through the defective nip do not have images thereon in a region thereof passing through the defective nip, the defective decoloring of the sheet does not occur and the location in the nip of the defect of the decoloring apparatus cannot be determined.
Therefore, it is preferable that a state of a heating device which decolors the image of the decolorable color material printed on the sheet by heating and pressing the image, be capable of being evaluated.